1) the temperature of the water stayed at a constant temperature just below 136° F
(58° C), where gypsum crystals form

2) the anhydrite dissolved into the warm water into it's component molecules of sulfate and calcium and then reassambled very, very slowly into gypsum crystals - called a phase transition.

What are the Crystals Like?

Scientists estimate that these selenite beams took hundreds of thousands of years to form. Gypsum is a fairly common crystal found throughout the world, but it is not highly valued because it is a very soft, fragile crystal. It has a tendency to flake off in sheets because of the loose atomic bonds between layers and the high water content of the mineral. If you note from the photographs these huge crystals have very, very sharply-defined edges, which means they are extremely sharp. Not all of the crystals in the cave are square pillars, some are shaped liked multi-faceted prisms, with some scientists describing crystals that are shaped like sharks teeth! These giant selenite crystals are also called 'moonstone' because they tend to reflect so much white light that they appear to glow like a full moon. When you first look at the breathtaking photos of the Cave of Crystals it appears as though they are full of giant ice crystals. Although the cave looks frosty cold because of the ice-like appearance of the crystals, it is very far from cool.

The conditions in the cave that caused the crystals to form also make it extremely dangerous for people. These caves have been completely filled with hot water for millions of years and were only recently emptied by people pumping water out of them for mining purposes. The temperature inside the Cave of Crystals is a staggering 150° F and the humidity is almost always 100%. Even though the water has been pumped out of the cavern, there is still enough groundwater seeping into the rock to be evaporated into steam inside the cave, making the crystals extremely dangerous to climb on because they are wet, slippery and sharp. The Naica volcano still receives heat from magma deep below the surface, which heats the entire interior of the mine. Scientists who have gone into the cave to study the crystals have developed highly specialized, protective suits to keep their bodies cool while they work, because without protection their brains would cease to function in about ten minutes at those high temperatures. Even with the protective suits, the scientists have a very limited amount of time to work in the cave before the ice in their suits melts and they have to leave to cool down their bodies.

What Will Happen to the Crystals?

The gypsum that forms these magnificent crystals is not stable in open air and has already begun to break down because of the absence of water from the cave. The crystals formed in water and in order to be preserved, need to be resubmerged in water where they can begin to grow again. And yes, they can get even bigger than they are now because there are no limits on the size of crystal growth. Scientists are working with mine operators to preserve these unique crystals by photographing them and discussing when, or if, they will reflood the caverns when mining operations are completed. There has been some discussion of providing air conditioning in the Cave of Crystals so it can be made more hospitable for people to be in there studying the crystals, and for admirers to visit the cave and see the crystals in person.